The present invention relates to a composition for a waterborne metallic-flake tint dispersion and a process of making the same. Specifically, it relates to tints which can be used alone, or more commonly in combination with other dispersions, as an air-dry or baking, low-VOC metallic-flake coating. Both immediate and long-term stability of metallic-flake tint dispersions are obtained.
It is well known in the art that various organic phosphate acid ester agents prevent or retard the reaction of water and aluminum flake, which otherwise may lead to the formation of potentially explosive hydrogen gas and the loss of desirable aesthetic effects. U.S. Pat. No. 4,350,535 disclose metallic-flake paste compositions which use such agents. U.S. Pat. No. 4,565,716 and U.S. Pat. No. 4,717,424 also describe such paste and coating formulations. In all these cases, however, the drawback is that the stability of the metal-containing paint is dependent upon how the metal paste or dispersion is mixed with the other paint components. While it is generally possible to obtain good aluminium stability and appearance by sufficiently stirring each component in individually, such appearance cannot be readily achieved when simply combining components in a can (without gradual or incremental mixing or stirring in of the second component) and then subsequently applying rejuvenation (mechanical shaking of the can) for a short time. The latter rejuvenation process is typical of automatic mixing-machine operations employed by automotive paint refinishers, especially in the U.S. The result of such a rejuvenation process has been aluminum flake which is unstable and not well incorporated into the liquid carrier. The product may even reagglomerate and flocculate, have poor aesthetics, or the metallic dispersion may separate from the rest of the paint components, which is visually perceived as an undesirable floating phenomenon or characteristic. Such defects may occur anytime, from immediately after rejuvenation until application and drying on a substrate. The magnitude of such a problem will vary depending on such factors as the presence of surfactants, solvent partitioning between the aqueous and particle phases, the shape and distribution of the metallic particles, surface tension, evaporation rate of solvents, and the temperature of the paint.
The present invention allows the user to formulate a metallic waterborne tint dispersion which can be used as a paint itself or can be combined with other (metallic or non-metallic) pigmented or non-pigmented waterborne dispersions to produce aqueous paints by rejuvenation or by mixing each component individually. The result is a paint which when sprayed on a substrate gives stable and reproducible appearance, commercial brightness, metallic two-tone, adequate humidity and water resistance, and dilutability with water as needed to adjust tinting strength, viscosity, and non-volatile content.